Antenna array feed system having dielectric phase compensators to adjust for phase lag differences



July l, 1969 H. R. PERINI ET Al. V3,453,636

ANTENNA ARRAY FEED SYSTEM HAVING DIELECTRIC PHASE COMPENSATOES TO ADJUST FOR PHASE LAG DIFFERENCES l Filed June 5, 1967 Sheet of 2 aimee-es T July 1, 1969 y H. R. PERINI ET AL ANTENNA ARRAY FEED SYSTEM HAVING DIELECTRIC PHASE COMPENSATORS United States Patent O ANTENNA ARRAY FEED SYSTEM HAVING DI- ELECTRIC PHASE COMPENSATORS T ADJUST FOR PHASE LAG DIFFEREN'CES Harry R. Perini, Astoria, John J. Stangel, Mahopac, Robert J. Timms, Glen Head, and Jack V. DAgostino, Massapequa, N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Filed June 5, 1967, Ser. No. 644,458 Int. Cl. H01q 21/00 U.S. Cl. 343-853 1 Claim ABSTRACT 0F THE DISCLOSURE Summary of the invention This invention performs the dual function of providing a feed system to a set of N-linear antenna arrays and of providing monopulse comparison. The feed system of this invention generates independent sum and difference illumination functions of any arbitrary shape for monopulse patterns. The feed system employs the shortest possible guide path to each radiating element thereby providing a low loss and light weight feed system.

The feed system of this invention can operate with phase Shifters for beam scanning. The small frequency dispersion which is normally compensated by the scanning phase shifters can be used for either frequency scanning or for generating two or more simultaneous sets of monopulse patterns.

An object of this invention is to provide a network that performs as both a feed system to a set of antenna arrays and a monopulse comparator.

Another object of `this invention is to provide a feed system which is light in weight and has a low insertion loss.

A further object of this invention is to provide a light weight, rigid structure having a compact form which is ideal for antenna packaging.

Brie]c description of the drawings FIGURE 1 illustrates an embodiment of the present inevntion;

FIGURE 2 shows a symmetrical cross-guide directional coupler;

FIGURE 3 depicts a schematic of the present invention; and

FIGURE 4 is a graph showing the variation of maximum coupling ratio with insertion loss for a linear array of N-elements.

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Description of the preferred embodiment A series fed network incorporating the features of the present invention is illustrated in yFIGURE l. The series fed network is constructed in the form of a rectangular waveguide grid with cross-guide directional couplers 1 at each junction. The coupling ratio at each junction is different and is derived from the illumination functions desired, number of cross arms, the insertion loss, and the relative position of the sum and difference ports, as will be explained later.

The series fed network of FIGURE 1 uses half-height rectangular waveguides vto enable operation in the lowest 15 percent of its recommended frequency range. The is the smallest size waveguide Iwhich can be employed and therefore the lightest possible waveguide feed system.

Small dielectric phase compensators 2 are placed in each cross arm 3 between the directional coupler -junction I1. At the end of each cross arm 3 are placed small polyiron slugs 4 to act as matched loads.

An output adapter 5 is incorporated at the end of each cross arm 3. Adapter 5 permits transformation to a coaxial line, strip line or waveguide depending upon the application.

For the best operating results, it has been found sum channel port `6 should be located at the end of cross arms 3 nearest to output adapters 5 and difference channel port 8 should be located at the end of cross arms 3 nearest to match terminations 4. Two matched terminations (not shown) are placed at the far end of long waveguides 7 and 9.

For antenna requirements, the spacing between cross arms 3 is approximately one half a free space wavelength. In orrder to simplify the illustration of FIGURE 1, only cross arms 3 that rest on top of waveguides 7 and 9 are shown. `In actual practice, a similar set of cross arms would be placed along the underside of waveguides 7 and 9 in staggered relationship to the top cross arms.

The exact characteristics of each coupler 1 are important in the analysis of the network operation and design. FIGfURE 2 shows the phase and :amplitude characteristics of a symmetrical cross-guide coupler. Energy passing through the main line of the coupler of FIGURE 2 has an amplitude, excluding normal. ywaveguide attenuation, which is reduced by a factor of \/l-C2, where C is the voltage coupler ratio. The two coupling holes provide a small reactance in the main line which results in a phase lag of --a with respect to the phase which exists in a straight piece of waveguide of `the same length. The phase lag -a is only a few degrees and represents a function of both the coupling ratio C and the relative frequency above the waveguide cut off frequency. The amplitude of the signal passing through the coupling holes is reduced by the factor C and the phase differs from an equivalent straight piece of waveguide by an angle --a. The relative angle between the two output arms is therefore `90 degrees out of phase.

The following procedure is employed to determine the coupling ratios at each junction in sum channel waveguide 7 of FIGURE l. The rst step is to cal-culate the coupling ratios for each junction in the sum channel.

This is a function of the sum illumination amplitude distribution, the number of cross arms, the waveguide attenuation and the proportion of power appearing in the end termination. A small amount of power must be dissipated in this end termination of waveguide 7, since there is an end coupler with a finite coupling ratio. In fact, the minimum power that can be dissipated in this end termination is determined by the tightest coupling ratio that can be practically built for cross-guide couplers and by the number of cross arms. The tightest practical coupling ratio for cross-guide couplers is approximately l db. A relatively easy ratio to achieve is 13 db, which is the design figure used in the example below.

The design proceeds by a trial and error method. The sum illumination function is arbitrarily assumed for a given number of cross arms. The proportion of power dissipated in the end termination is then assumed for the first trial calculation. The coupling ratios are then determined `from the assumed power division. If the tightest coupling ratio exceeds the practical limit for couplers, then a second trial calculation is made with a higher assumed power loss in the end transmission. The trials are repeated until the tightest coupling ratio approximately equals the practical limit. The result is an optimum design for the sum channel which has minimized the network insertion loss. The above described calculations can be carried out either by a machine calculator or a computer.

The design of the difference channel is more complex and is carried out by trial and error solution on a digital computer. The complexity results because the ditr'erence signal is fed to the antenna array ports by multiple paths, as is illustrated in FIGURE 3. This figure is a schematic representation of an antenna feed system having a sum channel 2 and a difference channel A equivalent to waveguide channels 7 and 9, respectively, of FIG- URE 1. This schematic represents three cross arms 31, 32, and 33 with outputs to the antennas 34, 35, and 36, respectively. 37, 38, and 39 represent the terminations 4 of FIGURE 1. This schematic also represents, above the broken line, the extension of the system to N cross arms and outputs. The waveguide terminations are represented at 40 and 41. Considering only the three cross arms 31, 32, and 33 with outputs 34, 35, and 36, a signal in the difference channel main arm will supply a portion of its strength at terminal 36 by following three paths. The output at terminal 36 is the resultant of the vector addition of the signal from the three paths. Two of the three paths make three right angle bends, while the other makes only one right angle bend. (A portion of the signal in A will make a right angle bend directly into cross arm 33 and on to output 36. Another portion will make a right angle bend into cross arm 32, another right angle bend out of the cross arm into sum channel E, another right angle bend out of the sum channel into arm 33 and on to the output 36. Still another portion of the signal will make a right angle bend from the difference channel A into cross arm 31, a right angle bend out of cross arm 31 into sum channel 2, and another right angle bend from the sum channel into cross arm 33 and on to output 36.) If the small phase lag a at each coupler were the same, then the signal in the path with one right angle bend would be 18() degrees out of phase with the signals coming from the paths with three right angle bends. This 180 degree phase relation can be achieved in practice, even Ithough each phase lag a is different, by inserting phase compensators in the cross arms between network junctions, as indicated by Q51, qbz, zpa, in FIGURE 3, and 2 of FIGURE 1. These phase compensators adjust for the small dilerences in phase lag between the various directional couplers.

The procedure for design calculation is based on the fact that4 all signal vectors at any one output port are either in phase or 180 out of phase. This makes possible the generation of arbitrary independent sum and difference illumination functions which track one another in phase at all output ports. The sum illumination is an even function about the array center with its peak atthe center. The difference lillumination is an odd function about the array center with zero value at the center. Therefore, in the difference channel network, a 180,'degree phase reversal is required between the two output terminals on either side of the network center. This phase reversal is achieved by adjustment of the phase compensators.

The procedure for calculation of the coupling ratios-ill the difference arm of the network is similar to the calculations in the sum arms. In this case, a desired arbitrary difference illumination function is assumed along with a power loss in the two end terminations of the main arms. A one volt signal is assumed to be fed into the difference port and the correct voltage is calculated for each output terminal and for the two end terminations. Once the voltages are determined, the coupling ratio of each coupler in the difference arm is calculated in turn, starting with the rst. The coupling ratios in the sum arm which were determined previously are used in this calculation. Again, if the tightest coupling ratio exceeds the practical limit, then the loss in the end terminations is increased for the second trial. The calculations are repeated until the optimum design is reached which minimizes the insertion loss, while remaining within the practical limits of coupler design.

FIGURE 4 is a graph showing the variations of maximum coupling ratio with insertion loss for linear array of N-elements. As can be seen from FIGURE 4, less power is dissipated in the end terminations for the larger number of outputs. The insertion loss is reasonably low for networks with sixty or more outputs, which corresponds to the needs of most antenna arrays.

The phase compensators are constructed from dielectric slabs placed inside the cross arms between the sum and difference channel waveguides. The desired dimensions are obtained from theoretical design curves on dielectric loaded waveguide.

We claim:

1. An antenna array feed and monopulse comparator system for providing sum and diierence signal coupling to a plurality of antenna array elements, the said system comprising:

(a) a half-height rectangular waveguide sum feed channel having termination means at one end of the waveguide and summation signal coupling means at the other end of the waveguide;

(b) a half-height rectangular waveguide difference feed channel having termination means at one end of the waveguide and dierence signal coupling means at the other end of the waveguide;

(c) the said summation and difference channel waveguides being positioned in parallel spaced apart relationship;

(d) a plurality of half-height rectangular waveguide cross arm elements in one-to-one correspondence with the said plurality of antenna array elements cooperating with and positioned perpendicular to and across the said parallel feed channels, the said cross arm elements having:

(l) -cross guide directional coupling means connecting each cross arm with each channel,

(2) antenna array element connecting means adjacent the said summation channel for connecting each respective cross arm to the respective antenna array element,

(3) waveguide termintaion means in each of the said cross arm elements adjacent the said difference channel,

(4) dielectric phase compensating means positioned inside each said cross arm element between the cross guide coupling to the said summation channel and the cross guide coupling to 6 the said diierence channel for adjusting the OTHER REFERENCES phase lag differences between the said cross Blass: Multidire-ctonal Antenna-A New Approach t0 gulde Coup.1mg mans l Stacked Beams, 1960 IRE Intl. Convention Record, PG (5 the spacmg between each Qf the sa1d-p1u1aon A&P, Man 21 24 1960 343 853 lty of cross arm elements belng approxlmately y one-half of the free space wavelength of the said 5 HERMAN K SAALBACH, Pfl-mm3, Exammeh signal.

` P. L. GENSLER, Assistant Examiner. References Cited Us. C. XR. UNITED STATES PATENTS 10 333 10I 31 1 3,255,450 6/1966 Butler 343-853 X 

